1. Field of the Invention
This invention relates to optical switches, and in particular to Y-branch digital optical switches.
2. Description of the Related Art
Digital Optical Switches (DOS) are increasingly replacing other types of optical switches in a wide variety of applications, including communications systems. One of the most popular digital optical switches is the Y-branch DOS that has received wide commercial acceptance primarily because of its robustness to variations in critical parameters, such as polarization, wavelength, temperature, and to a large extent, even the device geometrical variations. Typically, a Y-branch DOS is designed such that two waveguide branches intersect to define a Y-shape structure with a very small angle at the intersection of the branches. The composition of the waveguide structure may include a wide variety of materials, such as lithium niobate, and semiconductors, to name a few. A Y-branch DOS performs its switching function by adiabatically changing (i.e. slowly varying, as opposed to abruptly altering) the light propagation direction in one of the waveguides.
Specifically, switching in a Y-branch DOS is achieved by forcing a refractive index change in one waveguide branch with respect to the other. The change in refractive index may be induced by applying, for example, voltage and/or current to selected sections of the structure. Of particular significance among the characteristics of a Y-branch DOS is its step-like responses to applied voltage or current which allow the light to remain in a higher index branch, notwithstanding increases in the applied voltage or current beyond a given threshold. Hence, by operating the Y-branch DOS beyond some threshold value for applied voltage/current, the variations in polarization, wavelength, etc. mentioned above do not impact the switching capacity of the Y-branch DOS.
In spite of all the advantages offered by Y-branch DOS, certain shortcomings of those devices may prevent their use in certain applications. For example, the relatively high voltage drive needed to power lithium niobate-based Y-branch DOS limits their operative bandwidth since microwave power increases with applied voltage. Equally bothersome is the relative oversized length of the prior-art Y-branch DOS, which increases overall optical loss for these devices, and hampers their integration with other devices because of their longer "footprints". As mentioned above, the gradual and small changes in the light propagation direction to achieve the switching function in a Y-branch DOS dictate the small angle and long footprint structure (to avoid crosstalk) of the prior art Y-branch DOS. Prominent among the limitations of the small angle, long-footprint design of Y-branch DOS is the difficulty in the fabrication of these devices to use conventional photolithographic techniques to define the small (for example, less than 0.25 micron) separation at the vertex of the two waveguiding branches of the Y-branch DOS.
In an attempt to overcome the limitations of Y-branch DOS devices, Okayama et al., in an article entitled "Reduction of Voltage-Length Product for Y-Branch Digital Optical Switch", published in JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 11, NO. 2, 1983, pp. 379-387, proposed a two-angle shaped Y-branch DOS that provided enhanced crosstalk performance, reduced length and lower voltage drive for a Y-branch DOS. An example of the Okayama two-angle shaped Y-branch DOS is shown in FIG. 1, where the Y-branch DOS 10 has a first waveguide portion 12 connected to intermediate waveguide portions 14, 16 at a vertex 18, with each of intermediate waveguide portions 14, 16 associated with a first taper angle .theta..sub.1 and symmetrical about a longitudinal axis 20 of the first waveguide portion 12.
Each of second waveguide portions 22, 24 is respectively connected to the first waveguide portions 14, 16 and associated with a second taper angle .theta..sub.2. In this DOS 10, .theta..sub.2 &lt;.theta..sub.1 to form the tapered Y-branch configuration. Each of waveguide portions 12-16 and 22-24 may have identical widths w, and the configuration 10 may be symmetrical about the longitudinal axis 20.
Other prior art techniques modified the Okayama two-angle design to further reduce the length, the voltage/current drive and cross-talk degradation of a Y-branch DOS. Unfortunately, all the prior-art techniques do not change the compactness, and the voltage/current drive of a Y-branch DOS to an extent that significantly impacts the operative bandwidth of a Y-branch DOS. Equally lamentable is the lack of a design that would allow ease of fabrication of Y-branch DOS using conventional photolithographic techniques.